It is a web app for building websites. You don't write any code. You use their super slick interface for designing the site and managing the content.
They have just released "Squarespace 6", a kind of reboot to the app. I think it's much nicer than the previous iterations. In this latest version, the things that it makes sense to do while looking at a live preview of your site, you do while looking at a live preview of your site. The things that make more sense to do while looking at an admin screen of sorts, you do in an admin screen. I feel like previous iterations forced more into the live preview area.